I bought Rumble the day of his release, I was looking for a Build but I didn't find something as good as I expected to find here, so I decided for the first time to make my own Build...
I aim to make a killer tanky rumble. He has no tank spell so focused it will be hard for you, but HP and can help you to get out.

In runes I choose 9 , 9 to up your AP in the very early game, then 9 and 3 for the tanky aspect.

My masteries are obviously aggressive and bonus are welcome, also I put 2 points in Minions damages because of the reduce damage of fire on minions, the fire is very useful in early game to push enemies players under their towers, then you play safe (ward and no greedyness) and just wait for them to make mistakes to get a kill or two.

and should be use together in the very early, before minions spawn, very often in the grass enemies players are waiting for you to gank. Go together with you line mate, focus one with Ignite, then the first died, exhaust the second and kill him ^^.

Considering items I think my buy order is good, it's allow me to get many HP fast and AP too, even if you have to wait to get real OP Magic Damage.

In the end game when your buy order is finished do no hesitate to get Armor, this way you will be almost unkillable.

I

I use to harras with , if range enemies use your . Don't be affraid to use it 2 times, even if you can't hit the enemies hit the minions, you have no mana, be proud of it ^_- ! Moreover I've saw that using your 2 times or 1 time has no effect, it produce the same heat, and make the same cooldown, so spam it in early.

II

Then when you get your ulti, use it well, in teamfight do not go IN the team fight, stay at a good distance, when it begin use your ulti, you have a very very great range with it so use it, after that use to hit, in same time use your and then your .
At this moment if you did well with my build and some kills, you should have at least 2 kills.
I don't know why but people use to stay in the rumble's ulti, but that's great for you ^^'

III

Don't be afraid to use your often even in 1vs1, the more you have kills, the better it will be in real team fight. Also, if low life enemies are running away, use it to slow them (most of the time the itself kills them).

IV

For the invisible like Twitch, Evelyn, etc... If you know that they're near from you, use your , by moving and spinning in the area, this way you have good chance to harras them and launch a fight, the enemies gank is over.